In an AC induction motor, a rotating magnetic field is generated by stator coils to induce current flow and a magnetic field in the rotor. The interaction of the rotor's magnetic field and the stator's magnetic field provide torque to the rotor causing the rotor to rotate.
In order for the stator field to induce current in the rotor the rotor must rotate more slowly than the stator field. This slippage, which is a function of torque, causes the rotational speed of the AC induction motor to be unsynchronized with respect to the frequency at which the stator field is driven.
In contrast, in a synchronous motor, the rotor speed is synchronized to the frequency of the rotating stator field. In a permanent magnet synchronous motor, the rotor field is provided by permanent magnets eliminating the need for rotor windings or the induction of current between the stator and rotor.
When one uses a permanent magnet synchronous motor, it may be desirable to know the position of the rotor with respect to the stator. This position information may be required in order to commutate the stator field or may be used to track total angular displacement or velocity of the rotor for controlling a machine to which the motor is connected. For this purpose, it is known to attach the rotor to an optical encoder or resolver providing angular position information.
The use of such optical encoders or resolvers increases the size and cost of the motor.